Our objective is to assess how accurately patients can guess their INR result before it is obtained and to describe factors which inform their opinion of what their INR will be. In this prospective study, the investigators will enroll warfarin treated patients from 7 anticoagulation clinics in Iowa. Inclusion criteria are: age ≥18 years, warfarin use ≥60 days, INR goal of 2.0-3.0 or 2.5-3.5, expected warfarin use >6 months, and English speaking. Exclusion criteria include: use of self INR-testing, home INR draws, dementia, or residing in a long-term care facility. A data collection form will be completed prior to INR measurement for a 6 month period. Information will be collected for subject demographics, warfarin adherence, INR stability, INR prediction, and prediction rationale.